During the past year, research in the Molecular Immunology Section has been focused on the mechanisms of recognition by human CD8+ T cell antigen-specific receptors (TCRs). Analysis of TCR recognition has been examined using T cell functional assays plus kinetic and thermodynamic binding assays. A particular emphasis of these studies has been on the mechanism of how TCRs recognize the complex of MHC-bound haptenated peptides presented by HLA-A2. The overall conclusion is that there are conserved amino acids on the alpha one (K66) and alpha two (Q155) helices of the HLA-A2 molecule that are key anchor residues that are recognized by most HLA-A2-restricted TCRs, including those that recognize haptenated peptides. For most HLA-A2/peptide complexes whose structures have been solved, the side chains of these key amino acids also contact the bound peptide. These findings demonstrate that the molcular mechanism underlying MHC restriction is that the TCR/MHC interaction cannot be physically separated from the peptide/MHC interaction because key elements of the MHC molecule not only are directly contacted by the TCR but these same MHC elements also contribute to the conformation of the bound peptide. A crystal structure of the K66A mutant with a bound Tax peptide indicates that the bound Tax peptide has exactly the same conformation as when bound to wild-type HLA-A2. This surprising result indicates that the effect of the K66A mutation o on TCR recognition is due to TCR recognition of the part of the K66 residue that form TCR contacts, and not to effects on the conformation of the bound peptide. Analysis of TCRs that are specific for structurally diverse haptenated peptides reveals that many TCRs are capable of recognition of quite diverse haptens, but this cross-reactivity involves recognition of very similar components on the surface of the HLA-A2 molecule. We are now cloning and expressing these highly cross-reactive TCRs for kinetic and thermodynamic binding assays, and crystallographic studies. These studies will hopefully provide a biochemical and structural solution to the question of just how much plasticity is associated with TCR cross-reactivity.